1. Field of Invention
This invention relates to integrated circuits and in particular random number generators.
2. Description of Related Art
Random number generating circuits are well known. The most common of these circuits are based on the noise property of a biased semiconductor device. Some circuits use oscillators and rely on the natural variation in the frequency of slow oscillators to control the sampling of faster oscillators.
In U.S. Pat. No. 5,706,218 (Hoffman) a single, slow voltage controlled oscillator receives a noise input and controls the sampling of a set of ring oscillators. A circuit is used at the output of each ring oscillator to ensure that there is a near even distribution of logical ones and zeros in the random numbers. In U.S. Pat. No. 5,596,617 (Wolfe et al.) a feed back shift register of N stages has exclusive OR (XOR) circuits in the feedback logic and a gated clock which produces one blocked clock pulse in 2.sup.n clock pulses. In U.S. Pat. No. 5,327,365 (Fujisaki et al.) a parent processor in a parallel processor generates random number initial values and distributed the initial values to child processor elements which conduct processing to generate random numbers. In U.S. Pat. No. 4,905,176 (Schultz) a random number generator is disclosed which is based upon low frequency sampling of the output of a pseudo-random number generator that operates at a varying frequency from a free running ring oscillator. In U.S. Pat. No. 4,799,259 (Ogrodski) an array of oscillators operating at different frequencies is used as an input to an XOR network. The XOR network provides an output signal to a clocked D-type flip flop which samples the state of the random output signal from the XOR network. In U.S. Pat. No. 4,641,102 (Coulthart et al.) a five state ring counter is gated to a set of XOR circuits interposed between stages of a five stage shift register connected as a feedback shift register. A D-type flip flop with a low frequency clock and a high frequency data square wave controls the gating of the five stage ring counter to the set of XOR circuits.
Random number generators are useful in applications such as Smart Cards. It is therefore a requirement that random number generators be able to put into integrated circuits and utilize a relatively few components. It is also necessary that the output of the random number generator not be deterministic with a relatively short period as is the case with a simple linear feedback shift register where the feedback is XORed with certain bits in the shift register.